Requesting Legal Advice

Some months ago my sisterA asked if she could borrow my phone so she can use it as a means of communication for work. I agreed to lend her my phone for the time being. Following after that, i wanted to get back my phone since she already has an iphone5c. Since she frequently meets with my other sisterB, i asked her to pass the message about getting back my phone. I learnt that she did not want to give me back my phone and claims I gave it to her so now its hers. I made a post on her boyfriend's social media page (she blocked me on social media) stating her full name that she must return the phone to me with the hashtag #kawatanmoves and not to make her reputation worse. Now she and her boyfriend are threatening me that they will file a case against me.

1. What case can be filed? What grounds? Is it a strong case?
2. Is this qualified as theft? Can i file a case against her?

Estafa because there was "juridical transfer" of the thing. grounds: deceit, breach of trust and confidence, abuse of confidence.

It is a strong case IF your sister is not living with you in the same roof, Otherwise, the case is vulnerable for dismissal.